The present invention relates to an apparatus and method for reducing engine noise.
Four- and five-cylinder engines, used on many vehicles today, have inherently loud low frequency induction noise. The objectionable induction system noise on these engines typically occurs at frequencies of 250 Hertz or less. For example, a four-cylinder engine operating at 3000 revolutions per minute (RPM) generates induction system noise at its fundamental firing frequency: 100 Hertz (Hz). This is twice the frequency of the engine RPM, and is accordingly termed xe2x80x9csecond-order xe2x80x9d noise. The range of expected induction noise frequencies is based on the expected engine speeds. In the case of a four-cylinder engine, operating at 1500-6000 RPM, the second-order induction noise frequencies range from 50-200 Hz. Larger engines typically generate induction noise of somewhat higher fundamental frequencies, but even these frequency ranges are relatively lowxe2x80x94e.g., a five-cylinder engine generates 2.5 order noise, which at 3000 RPM is 125 Hz.
Various methods are employed to reduce the induction system noise, the most common of which is to use a Helmholtz resonator on the engine air intake pipe. A Helmholtz resonator includes a chamber having a small opening, typically a tube or xe2x80x9cneckxe2x80x9d. The neck is connected to the engine air intake pipe, since it is this pipe through which much of the engine noise escapes. The sound waves (noise waves) generated by the engine travel along the intake pipe where their acoustic pressure impinges on the resonator opening. This acoustic pressure excites a mass of air in the resonator neck, causing high acoustic pressures within the resonator chamber at the resonant frequency. The chamber acoustic pressure reacting back against the air mass in the neck produces out-of-phase acoustic pressures at the intake pipe to cause full cancellation of intake noise at the resonant frequency. In this way, much of the engine noise is eliminated as the out-of-phase acoustic pressures in the intake pipe cancel each other. In order for there to be complete cancellation, the incident and reflected acoustic pressure frequencies must be equal; otherwise, there is only partial cancellation and unwanted noise escapes the intake pipe. In addition, even if the incident and reflected acoustic pressures are of equal frequency, the level or amplitude of the reflected pressures may not be enough to cancel those of the intake pipe noise waves. The frequency at which the attenuating acoustic pressures reach their maximum amplitude is known as the resonant frequency.
Because the amplitude of the attenuating acoustic pressures is very high at the resonant frequency, it is possible to attenuate even very loud noise at this frequency. One of the limitations of a Helmholtz resonator is that it operates at a single, fixed resonant frequency. Thus, very loud noise at other frequencies is not as effectively attenuated. This means that although noise not at the resonant frequency can be reduced, it cannot be completely canceled if it is very loud. The resonator attenuating acoustic pressures do have a range of frequencies, known as bandwidth, over which they operate. They are most effective at the resonant frequency, where their amplitude is highest. The effectiveness of the attenuation quickly tapers off, however, at frequencies on either side of this peak.
A number of parameters determine the resonant frequency and bandwidth of a Helmholtz resonator, including the chamber volume and the neck length and neck area. In general, increasing the chamber volume increases both the magnitude of the attenuation (the amplitude of the attenuating acoustic pressures) and the bandwidth. It is impracticable to increase the chamber volume beyond a certain size, since vehicle packaging requirements limit the available space. Increasing the cross-sectional area of the neck is another way to increase the magnitude of the attenuation; however, the neck area is typically limited by the diameter of the intake pipe into which it connects.
An often undesirable effect of increasing the neck area is that the resonant frequency is increased such that it no longer equals the frequency of the engine noise being targeted. To counteract this resonant frequency shift, the neck length is increased to drive resonance back to the target frequency; however, longer necks can narrow the resonator""s attenuation bandwidth. The neck length is, of course, constrained by packaging requirements, so the ability of a designer to vary this parameter is also limited. Moreover, even if all of the parameters are optimized such that the resonant frequency cancels the targeted engine noise frequency, the bandwidth of attenuation is often too small to adequately reduce other noise frequencies generated at different operating speeds.
One way to deal with the limitations inherent in Helmholtz resonators is to create an xe2x80x9cactive xe2x80x9d resonatorxe2x80x94i.e., one that changes certain parameters as engine operating conditions change. One such example is found in U.S. Pat. No. 4,546,733 issued to Fukami, et al. on Oct. 15, 1985. Fukami teaches an induction system resonator having a rotary switch valve driven by an actuator that is controlled by a computer. As the engine speed changes, the predominant frequency of the noise is calculated. The computer outputs a driving signal to the actuator, which in turn rotates the rotary switch valve to appropriately adjust the resonant frequency of the resonator. Although active resonators such as this have the advantage of a variable resonant frequency, they are much more complex and therefore much more expensive than fixed frequency resonators. Thus, neither an active resonator, nor the current Helmholtz resonators, provide a low cost solution to the problem of attenuating induction system noise at a range of engine speeds.
Accordingly, it is desirable to provide a simple, low cost noise attenuation apparatus which overcomes the shortcomings of the above-referenced prior art by providing attenuation of sufficient magnitude at a pre-fixed resonant frequency, and a bandwidth of sufficient range, to adequately attenuate engine noise without the need for active, computer driven controls. It is also desired to provide a method for tuning the apparatus to target a specific noise frequency, while increasing the bandwidth over that of conventional Helmholtz resonators.
One aspect of the present invention provides an improved fixed frequency noise attenuation apparatus that has a higher magnitude of attenuation than conventional fixed-frequency resonators.
Another aspect of the invention provides an improved noise attenuation apparatus with a large bandwidth to effectively attenuate engine noise over a range of frequencies, without using active, computer driven controls.
A further aspect of the invention provides a method of tuning the improved noise attenuation apparatus by varying certain dimensional parameters of the apparatus, such that noise attenuation is optimized for a particular application.
Accordingly, a noise attenuation apparatus for attenuating noise from an engine having a fluid-carrying conduit is provided, which includes a resonator chamber and a plurality of connecting tubes adapted for disposition between the fluid-carrying conduit and the resonator chamber for facilitating sound transfer between the fluid-carrying conduit and the chamber.
In another aspect of the present invention, the fluid-carrying conduit is an air intake pipe, and the plurality of connecting tubes are two cylindrical necks, normal to the intake pipe and parallel to each other.
It is a further aspect of the invention to provide an induction system resonator for attenuating noise from an engine having an intake pipe that comprises a resonator chamber and a plurality of connecting tubes of approximately equal length. Each tube includes an intake end adapted for opening into the intake pipe, and a chamber end opening into the resonator chamber.
In yet another aspect of the invention, a method of attenuating noise from an engine having a fluid-carrying conduit using a noise attenuation apparatus, includes providing a resonator chamber with a generally cylindrical first tube disposed between the resonator chamber and the fluid-carrying conduit for attenuating the engine noise. The method further includes tuning the noise attenuating apparatus to target specific engine noise frequencies by providing the resonator chamber with a generally cylindrical second tube having a length approximately equal to the first tube, and disposed between the resonator chamber and the fluid-carrying conduit at a certain distance from the first tube.
The above object and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings.